1. Field of the Invention
The present invention relates to digital color printing in which black objects and color objects are intermixedly printed, and more particularly, in which the composition of the black color for the black objects is automatically changed depending on the presence or absence of background color which contains non-black colorants.
2. Description of the Related Art
Digital color printers use a plurality of colorants, such as cyan, magenta, yellow, and black, to form images which appear to the human visual system to contain a multitude of colors In particular, the color black may be formed of equal or nearly equal combinations of cyan, magenta, and yellow. In practice, the black colorant is also often added to the combination of cyan, magenta, and yellow colorants to increase the maximum density and avoid problems of unwanted color. When black is formed with a mixture of some of cyan, magenta, yellow, and black, it is known as process black. The color black may also be formed by using the black colorant alone. In this case, it is known as single-color black.
Each type of black, process black and single-color black, have advantages and disadvantages when used in different contexts in the printing process.
When single-color black is used over a color field, it is often the case that the single-color black colorant has a gloss which is different from the gloss of the surrounding color field, and the gloss difference can be objectionable. Further, the maximum density of the single-color black colorant is often not as great as the maximum density obtainable with process black, and the density difference is particularly noticeable on certain color backgrounds. Also, the ink coverage or toner pile height of single-color black may be noticeably less than that of the surrounding color background. Finally, the presence of misregistration between the plurality of separations during the printing process, or the presence of other interactions between the separations, can result in objectionable white or light-colored fringing around a single-color black object on a color background. This occurs for example if the single-color black colorant is not printed precisely and without distortion into the hole left at that position when printing the other separations.
On the other hand, when process black is used in a white or grey field which does not contain any non-black colorants, slight misregistrations or interactions among the plurality of separations can result in objectionable color fringing at the edges of the object. Furthermore, the use of multiple colorants to form process black can result in blurred edges, thickened objects, and objectionable amounts of ink or toner coverage.
These advantages and disadvantages of single-color and process black are known in the graphics arts industry, and knowledgable graphic artists attempt to design pages which utilize -the advantages and avoid the disadvantages. However, some situations, such as a black object placed only partially over a color field, or a color background which changes rapidly, or complex text or graphics over a complex color background, makes it difficult to avoid the printing problems associated with mixedly printing black and color. Furthermore, many pages not designed by a knowledgeable graphic artist can benefit from a method and apparatus to automatically create a background dependent black image.
U.S. Pat. No. 4,700,399 to Yoshida discloses a color image processing apparatus which has an edge detector and a control unit for controlling a black reproduction quantity for an edge according to an output from the detector. For edges, densities of color signals for yellow, magenta, and cyan are reduced in accordance with the edge quantity to increase black density.
U.S. Pat. No. 4,953,015 to Hayasaki et. al. discloses a method for printing a color image which makes it possible to obtain a higher density black. Black ink is placed first, and then a plurality of other inks such as cyan, magenta, and yellow are allotted according to a color matrix table and are superimposed on at least a portion of the black ink.
U.S. Pat. No. 5,241,396 to Harrington discloses a method for printing a color image which yields dense black images without thickened, blurred edges. A modified black bitmap is produced by eroding the edges on the original black bitmap. The modified black bitmap is used to instruct the printing of at least one other colorant and then the original black bitmap is used to print the black colorant to yield a dense black image.
These methods have in common the attempt to limit the blurring disadvantages of process black on a white background so that it can be used in place of single-color black. Instead of that approach, the current invention describes a printing method which uses knowledge of the background color to choose whether to print single-color black or process black in order to utilize at the appropriate time the advantages of each type of black.